Computational studies of the molecular recognition of halide anions by calix[4]aromatics

Monte Carlo statistical mechanics simulations have been carried out to elucidate the complexation of halide anions by a bis(phenylurea) calix[4]arene and by a calix[4]pyrrole. Optimized structures for the complexes in the gas phase are found to be maintained in chlorocabon solvents, though there are reorganization penalties of 20-30 kcal/mol for the hosts to achieve the binding geometries. All complexes feature coordination of the anions through four hydrogen bonds. The binding affinities decrease with increasing size of the anion. It is demonstrated that fluoride affinities are underestimated experimentally owing to difficulties in obtaining anhydrous tetraalkylammonium fluoride salts.